Chemical imaging has a powerful capability for material characterization, process monitoring, quality control and disease-state determination. Chemical imaging combines chemical analysis with high-resolution optical imaging including optical spectroscopy, Raman imaging as well as fluorescent and IR techniques.
Raman effect is a phenomenon in which a specimen scatters incident light of a given frequency into a spectrum which has lines caused by interaction of the incident light with the molecules making up the specimen. Different molecular species have different characteristic Raman spectra. Consequently, the Raman effect can be used to analyze the molecular species present in the sample. Raman chemical imaging provides molecular-specific image contrast without the use of stains or dies. Raman image contrast arises from a material's intrinsic vibrational spectroscopic signature which is highly sensitive to the composition and structure of the sample as well as its local environment. As a result, Raman imaging can be performed with little or no sample preparation and is widely applicable for material research, failure analysis, process monitoring and clinical diagnosis.
Compared to conventional non-imaging systems, a chemical imaging system based on a tunable filter enables visualizing the distribution (morphology and architecture) of chemical species in heterogeneous samples with molecular compositional specificity. Raman images can be collected non-invasively with limited or no sample preparation, at high spatial resolution, and with high fidelity where the spatial fidelity is limited by the number of pixels of a charge coupled device (CCD) detector. Most importantly, every image pixel has associated with it a Raman spectrum whose quality is comparable to that obtained with conventional non-imaging spectrometers. Chemical imaging simultaneously provides image information on the size, shape and distribution (the image morphology) of molecular chemical species present within the sample.
In order to acquire 3D data sets in Raman imaging systems, the two dimensions of the image are recorded directly by a CCD camera while the multispectral information is acquired by capturing images at discrete wavelengths selected by the tunable filter, e.g., a liquid crystal tunable filter or LCTF.
In general, LCTF is an electro-optically controllable spectral bandpass filter which can function from the visible region to the near IR with a continuously tunable wavelength. In an imaging system, an LCTF is free of image shift with tuning. However, LCTFs have noticeable drawbacks. For example, LCTFs have a low peak transmittance. In addition, LCTFs are susceptible to thermally-induced drift in their spectral bandpass. In theory, the LCTF is free of optical distortions and spectral leakage; but in reality these defects always exist. Finally, LCTF systems are costly. Therefore, it is desirable to address the signal loss (and, hence, loss of efficiency) issues associated with LCTFs.